Emmet was a man of liberal political sympathies and became involved with campaign to extend the democratic franchise for the Irish Parliament and to end discrimination against Catholics. He was called to the Irish bar in 1790 and quickly obtained a practice, principally as counsel for prisoners charged with political offenses. He also become the legal adviser of the Society of the United Irishmen.

When the Dublin corporation issued a declaration of support of the Protestant ascendancy in 1792, the response of the United Irishmen was their non-sectarian manifesto which was largely drawn up by Emmet. In 1795 he formally took the oath of the United Irishmen, becoming secretary in the same year and a member of the executive in 1797. As by this time the United Irishmen had been declared illegal and driven underground, any efforts at peaceful reform of government and Catholic emancipation in Ireland were abandoned as futile, and their goal was now the creation of a non-sectarian Irish republic, independent from Britain and to be achieved by armed rebellion. Although Emmet supported this policy, he believed that the rebellion should not commence until French aid had arrived, differing from more radical members such as Lord Edward Fitzgerald.

British intelligence had infiltrated the United Irishmen and managed to arrest most of their leaders on the eve of the rebellion. Though not among those taken at the house of Oliver Bond on the 12th of March 1798 (see Lord Edward Fitzgerald), he was arrested about the same time, and was one of the leaders imprisoned initially at Kilmainham Jail and later in Scotland at Fort George until 1802. Upon his release he went to Brussels where he was visited by his brother Robert Emmet in October 1802 and was informed of the preparations for a fresh rising in Ireland in conjunction with French aid. However, at that stage France and Britain were briefly at peace, and the Emmets' pleas for help were turned down by Napoleon.

He received news of the failure of Robert Emmet's rising in July 1803 in Paris, where he was in communication with Napoleon Bonaparte. He then emigrated to the United States and joined the New York bar where he obtained a lucrative practice.

His abilities and successes became so acclaimed and his services so requested that he became one of the most respected attorneys in the nation, with United States Supreme Court Justice Joseph Story declaring him to be "the favourite counsellor of New York." [2] He argued the case for Ogden in the landmark United States Supreme Court case of Gibbons v. Ogden, 22 U.S. 1 (1824) relating to the Commerce and Supremacy clauses of the United States Constitution.

Thomas Addis Emmet is the father of prominent New York jurist and Irish American activist Robert Emmet (born in Dublin), grandfather of another prominent New York jurist and attorney general, Richard Stockton Emmet,[5] and great-grandfather of the notable American portrait artist sisters Rosina Emmet Sherwood, Lydia Field Emmet and Jane Emmet de Glehn,[6] as well as their first cousin Ellen Emmet Rand. Rosina's twin brother was West Point graduate and Medal of Honor winner Robert Temple Emmet.[7] He is the great-great-grandfather of the playwright Robert Emmet Sherwood. His grandson, Dr Thomas Addis Emmet, a prominent doctor and Irish American activist, requested that he be re-buried in Ireland so he could "rest in the land from which my family came." Dr Emmet is interred in Glasnevin Cemetery in Dublin, the final resting place of many of Ireland's patriots.[8] His grave marker was designed by the father and brother of the revolutionary Patrick Pearse.

Thomas Addis Emmet (grandson, 1828–1919), Ireland under English rule, or a Plea for the plaintiff [ With the Diary of Thomas Addis Emmet (1764–1827, grandfather), while acting in Paris as the secret agent of the United Irishmen, from May 30, 1803, to March 10, 1804 ]. New York City, G. P. Putnam, 1903 (first print, see the online catalogues of the Library of Congress or the Bibliothèque nationale de France).